ath9k_hw: restrict valid nf readings for AR9271 to -114

Noisefloor values read on AR9271 are unreliable if they
are less than -114, set those statically to -116.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>